Procedure and means for measuring with the aid of a radio-isotope source the distribution of fillers in a web

ABSTRACT

A procedure and means for non-destructively measuring the distribution of the filler and/or coating materials in the thickness direction of paper or cardboard. Radiation from a radio-isotope source is used to excite in the material component its characteristic X-ray radiation, the intensity of this radiation being observed. Measurements are made on both sides of the paper and the contents of other filler components are also determined by X-ray absorption in order to eliminate the effects of these components disturbing the distribution measurement. The base weight of the paper is measured e.g. by beta radiation absorption. Measurements are made both by measuring the characteristic radiation of the material components excited in the paper with different radiator sources and with the aid of absorption measurements of radiation directly from the source or produced with its aid in transformation targets, thus eliminating the effects of these components.

BACKGROUND OF THE INVENTION

The present invention concerns a method for measuring the distributionin the thickness direction of filler and/or coating materials of paper,cardboard or the equivalent, and the contents of said materials withoutdestroying the sample. In the method or procedure of the invention, theradiation emitted by an x-ray source is used to excite in the materialcomponent under examination of the object under measurement itscharacteristic x-ray radiation and the intensity of this radiation isobserved. In procedure, measurements are carried out on both sides ofthe specimen under examination. In addition, the contents of otherfiller components are determined by x-ray radiation absorptionmeasurements in order to eliminate the effects of such componentsinterfering with the distribution measurement, and the base weight ofthe paper is determined by beta radiation absorption measurement, or byanother equivalent procedure.

Furthermore, the present invention concerns apparatus for applying themethod and novel uses of the procedure and the apparatus.

When paper and paper machines are discussed in the following, referenceis generally made both to paper and cardboard, and respectively both topaper and cardboard machines.

Fillers, which as a rule are mineral substances, are incorporated in thepaper primarily for their effect of improving the printing technologicalproperties. Fillers are most commonly used for printing papers. Thefiller addition improves the opacity, lightness, printer's inkabsorption and surface smoothness of the paper.

The fillers influence in a particularly advantageous manner the qualityof paper to be glazed.

It is known in the art to add filler material in two ways, either bymass filling or by coating. In the mass filling method or procedure, thefiller material is added in the form of suspension to the pulp sludgebefore the arrival of the sludge on the paper machine, whereby thefiller material is admixed with the entire fiber material in thefinished paper. In the coating procedure, a suitable glue substance isadmixed with the filler material in the aqueous phase, such as starch orcasein, whereafter the surface of the paper is brushed with this mixturein a continuous process.

The filler materials in paper tend to be non-uniformly distributed inthe thickness direction of the paper, causing one-sidedness of thepaper. The one-sidedness of paper manufactured on Fourdrinier machinesis due to the fact that the fillers are washed out together with thewater that is drained, from the lower part of the pulp web into thedrainage water, whereby they become enriched in the upper part of theweb. As is known in the art, efforts have been made to reduce theproblems of one-sidedness, not only by additives improving theretention, but also by gentle dewatering at the initial draining phase,which requires a longer dewatering time and therefore implieslengthening the wire section or reducing the speed of the paper machine.

In machines with a planar wire, the difficulties with the fines andfiller distribution manifest themselves when papers for offset printingare manufactured. A high filler and fines content on the top surface ofthe paper causes dusting, which is a serious detriment in the offsetprocess. In contrast, papers manufactured on a twin wire machine areconsidered well appropriate for offset printing. This is due to thesymmetrical shape of the fines distribution and to equal leaching ofboth surfaces of the web due to two-sided dewatering. It is in factgenerally held that due to more uniform fines distribution, the printingby offset on paper manufactured on a twin wire machine is moresuccessful than that on paper manufactured on a Fourdrinier machine.Offset printability has indeed increased in significance because offsetprinting is increasingly replacing the letterpress printing procedure.

On the other hand, the filler content of the surfaces of the paper webcannot always be brought to a desired level with a twin wire former.Even when planar wires are used only the top side of the web (the sidefacing away from the wire) is satisfactory as to its filler content. Thelow filler content of the web surface is particularly problematic inso-called SC gravure papers. Attempts may be made to increase the fillercontent of the paper surfaces by increasing the filler content of thepulp in the headbox, but even with this expedient, a satisfactorycondition is not achieved because of the above-mentioned poor retentioncharacteristic of the filler and of its enriching in the inner parts ofthe paper. In addition, when the filler content in the headbox has to beincreased, the consistency in the headbox is likely to become excessiveso that it impairs the formation of the paper.

Modern high-speed printing presses impose particularly high requirementson the printing paper. These requirements are based on trouble-freeoperation of fast printing presses and on the appearance of theprinting. The imprint is considerably influenced by the symmetry betweenthe sides of the paper and the quality of the surfaces of the paper,which is naturally also influenced by the distribution of the fillers.Heretofore no methods or procedures and apparatus have been in use withwhich the filler distribution could have been measured even on lineeither in the paper machine, in the printing press or in the papercoating means.

It is known in the art, as described in Finnish Pat. No. 40587,inventors Juhani Kuusi and Antti Lehtinen, applicant Valmet Oy, toexcite the characteristic x-ray radiation of the filler material byradiations such as alpha, beta, gamma or x-ray radiation, penetrating tovarious depths in the paper, and in this ways to gain information on thevertical distribution of the filler. The procedure has been described ingreater detail in a paper by J. Kuusi, entitled "Determination ofContent and Distribution of Filler and Coating Materials in Paper UsingRadioisotope X-Ray Spectrometry", Paper and Timber No. 4A 1970. As wasobserved in the paper, variations in relation to each other of thefiller contents cause certain effects of which the quantitativeelimination by the procedures described in the paper is impossible. Thishas impeded the introduction, practice of such procedures.

The state of the art regarding filler measurements is illustrated ingeneral by a publication of April, 1982 by Buchnes A. McNelles L. A. andHewitt J. S., entitled "The Application of X-Ray Absorption andFluorescence Analysis to the Measurement of Paper Additives", Int. J.Appl. Radiat. Isot. Vol. 33, pp. 285 to 292 (1982), where a fluorescenceand absorption technique is used for determining the total contents ofdifferent fillers, based on the assumption that the fillers areuniformly distributed in the thickness direction of the paper. Inpractice, this is hardly ever the case. It is thus understood that inthis application, and the references cited therein and in its author'spatent for "On-Line System for Monitoring Sheet Material Additives",U.S. Pat. No. 4,081,676, March, 1978, no endeavors whatsoever were madeto determine the important thickness-direction distribution, nor has iteven been taken into account as a potential source of error indetermination of the total filler content. It should be noted, however,that in the instances described in the paper, the influence of saidsource of error is minimal.

Procedures or methods capable of determining the filler distribution andthe total filler content directly in the paper machine are not in use atall.

SUMMARY OF THE INVENTION

The principal object of the invention is to provide a new method orprocedure and apparatus suited, in addition to laboratory measurementsfor the measurement of filler distribution, which method and apparatusmake possible the control and adjustment of the manufacturing process ina paper machine on the basis of filler distribution measurements.

An object of the invention is to provide a method and apparatus fordetermining the thickness-direction filler distribution in the paper andthe total filler contents either in the laboratory or directly in apaper machine, on line.

Another object of the invention is to provide a procedure and apparatusfor determining the thickness-direction filler distribution in the paperand the total filler contents when the contents of different fillercomponents, such as, for example, CaCO₃, TiO₂, kaolin, talc orequivalent, are variable.

Still another object of the invention is to provide an opportunity notonly for immediate product quality control directly in the machine, online, but also an entirely new possibility of controlling the papermanufacturing process, the significance of which is emphasized whenefforts are made to manufacture printing paper meeting ever greaterquality requirements at lowest material costs. Yet another object of theinvention is to achieve, measure and control the distribution, whichprovides an opportunity to develop the paper machine construction andthe total control system of paper machines.

Another object of the invention is to provide a method which is suitablefor quality control of the paper fed into fast modern printing presses,and possibly for the control and/or adjustment of the operation ofprinting presses.

To achieve the aims presented in the foregoing and those which willhereinafter become apparent in the method of the invention, thedistributions of fillers and equivalent are determined by combinedprocessing of the two following sets of measurements.

1. Absorption measurements for determining the contents of differentfiller components with radiation obtained directly from the source orproduced with its aid in appropriate transformation targets; as manymeasurements as there are filler components to be considered separateones.

2. Measurements of the characteristic radiation of the materialcomponents excited in the paper by different sources of radiation.

In the apparatus of the invention a measuring head comprising radiationsources and transfer mechanisms therefor, a radiation transformationplate or plates and a transfer mechanism therefor, and a radiationdetector and a pre-amplifier. The measuring head is connected to ameasuring device having a power source, an amplifier and a counter,processor and display unit.

A second embodiment of the apparatus of the invention comprises ameasuring head with an x-ray source emitting radiation varying in energyduring the measuring cycle in a known manner, and radiation detectorsand pre-amplifiers. The measuring head is connected to a measuringapparatus comprising power sources, amplifiers and a multi-channelcounter, processor and display unit using a time axis.

The method or procedure and apparatus hereinbefore described are used,as taught in the invention, for example in a paper machine, in on-linemeasurement for measuring the filler distribution in the thicknessdirection and the total filler content of paper. In addition, theobtained measurement may be used as feedback signals in the controlsystem of the paper machine, in the control of the filler distribution,and/or of the total filler content of various filler materials. Anadvantageous application of the invention is in measurement, andpossibly in the control, of the coating material content and/or coatingmaterial distribution in paper or cardboard that is either being coatedin an on-line process, or has been tested in a separate coatingapparatus, in particular of its one-sidedness.

One potential application of the invention is the quality control of thepaper being fed into a printing press and/or governing, and possiblycontrolling, the operation of the printing press.

As has in part become apparent from the foregoing, the inventive idea isthat the intensity of the characteristic x-ray radiation of the fillercomponent excited with different radiation sources, and possibly withdifferent angles of incidence of the exciting radiation, is measured onboth sides of the paper. This intensity provides information about theshape of the distribution. In addition, in this x-ray fluorescencemeasurement it is possible to determine, the intensity of the excitingradiation scattered back from the paper and which correlates, forexample with the base weight of the paper. What is significant from thepoint of view of practical applications is that the contents of variousfiller components are measured by x-ray absorption measurements. Thesemeasurements make use of the primary radiation emitted by a radiationsource and a radiation with desired absorption properties which has beenderived from this source, or from a source placed on the other side ofthe paper, with the aid of appropriate transformation targets. Theauxiliary quantity is the absorption measurement signal of betaradiation used as routine in measurements on paper for determinations ofbase weight in g/m² (fibers plus filler). Based on the results of theabsorption measurements, it is possible by calculation to eliminate theeffects of the variation of the different filler components' contents onthe fluorescence measurements, and in this manner to determine thefiller distribution and the contents of different filler components.

In the laboratory, the invention affords an opportunity for rapidquality control of the paper, and thereby for the control of themanufacturing parameters with a given lead time. Particularly the fillerdistribution close to the surface layers of the paper has considerablesignificance concerning the printability of the paper. Furthermore, adistribution of proper shape provides an opportunity to use filler inabundance, thereby lowering the total material costs. The procedures ormethods presently used in laboratories, such as dividing the paper intodifferent layers by a tearing tape, incineration of layers and ashdetermination, are slower by one order of magnitude and more inaccuratethan the method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference is had to thefollowing description, taken in connection with the accompanyingdrawings, in which:

FIG. 1 is a graphical representation of a typical filler distribution inpaper manufactured on a Fourdrinier machine;

FIG. 2 is a graphical representation of mass absorption coefficients ofsome mineral filler and coating materials of paper and of water, ofcellulose for low energy x-ray raidation;

FIG. 3 is a schematic diagram illustrating the main principle of thefluorescence measurement of the invention;

FIGS. 4A and 4B are schematic diagrams presenting the principle of thefluorescence measurement of the invention with two different angles ofincidence of the exciting radiation and angles of departure of theexcited radiation;

FIG. 5A is a graphical representation of the distribution of fillercomponents prior to coating the paper; FIG. 5B is a graphicalrepresentation of the same paper after the coating process.

FIG. 6A is a schematic diagram of an embodiment of the apparatus of thefluorescence measuring apparatus of the invention;

FIG. 6B is a schematic diagram of a second embodiment of the apparatusof the invention for absorption measurement with x-ray radiation;

FIG. 6C is a schematic diagram of a third embodiment of the apparatus ofthe invention for absorption measurement with beta radiation; and

FIG. 6D is a schematic diagram of a fourth embodiment of the inventionalternative to FIG. 6C for absorption measurement with x-rays.

FIG. 7 is a schematic diagram of a measuring head disposed on atransverse measuring beam for reciprocating movement over the paper web.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A typical filler distribution of paper in its thickness direction x isshown in FIG. 1. The filler is least in quantity on the wire side. Inthis instance, the maximum is reached slightly above the center-point ofthe paper, marked 0.5 on the horizontal axis. The filler contentdecreases towards the top surface (x=1).

The attenuation or extinction of x-ray, gamma and beta radiation inmatter can generally be expressed by the expotential formula:

    I=I.sub.o e.sup.μm,

where I (1/s) is the intensity of the radiation that has gone through amass course m (g/cm²), I_(o) (1/s) is the original intensity of theradiation and μ (cm² /g) is the absorption coefficient representing theextinction or attenuation capacity of the material.

The absorption coefficients for low energy (1 to 10 keV) x-ray and gammaradiation of material which are important in view of filler measurementsare set forth in FIG. 2, plotted over energy. Both are the same type ofelectromagnetic radiation. In FIG. 2, the abscissa represents the energy(in keV) and the ordinate represents the absorption coefficient μ (incm² /g). With the exception of a few discontinuous irregularities, theabsorption coefficient and therefore also the attenuation in thematerial, decreases with decreasing energy. However, some of thediscontinuous jumps seen in FIG. 2 are of central importance in theembodiments of the invention. If the graph of the absorption coefficientof calcium carbonate (CaCO₃) is scrutinized, we find that it dependssmoothly throughout the range from 1 to 4 keV, until, at 4.04 keVenergy, its value discontinuously increases to be tenfold and thereafteronce more decreases smoothly with increasing energy of the radiation.The physical cause underlying this jump is that in the range underconsideration, x-ray and gamma radiation are attenuated in the materialin the manner that the energy of the radiation quanta transfers totallyto electrons in the atoms. Such electrons, by virtue of the energyimparted to them, are flung out from the atom, leaving behind a vacancyin the electron shroud. The energy of the x-ray or gamma quantum has tobe higher than the binding energy holding the respective electron to itsatom. When the energy of the radiation is lower than the 4.04 keVcorresponding to the jump in the graph for CaCO₃, the radiation is notable to detach the electrons of its inner shell (the K shell), which arethe electrons most strongly bound to the atom from the calcium atom.When the energy of the incident radiation surpasses this limit, itsquanta can become absorbed in the substance by detaching electrons fromthe inner shell, and this exactly gives rise to the discontinuousincrement of the absorption coefficient. The higher the atomic number ofa substance, in practice usually the heavier it is, the higher is theenergy at which is found this K absorption limit, that is the absorptionlimit corresponding to the K shell.

Thus, it is hown in FIG. 2 that the K absorption limit, due to titanium,of titanium dioxide (TiO₂) is located at an energy of 4.96 keV. In talcand kaolin, the element with the highest atomic number is silicon (Si),and therefore the absorption coefficient decreases steadily after theabsorption limit of silicon at 1.8 keV with increasing energy of theradiation.

It is thus understood that when radiation having an energy higher thatthe K absorption limit of calcium is directed on a substance, forexample calcium, vacancies will form on the inner electron shells of theatoms. When these are filled by electrons falling from outer shells, thesubstance emits its characteristic K x-ray radiation, the energy ofwhich because of recoil losses is slightly lower than the energy of theK absorption limit. The strongest line of the calcium K has energy 3.69keV, which has also been indicated on the energy axis in FIG. 1.

The characteristic x-ray radiation of each element produced throughabsorption is utilized in a manner known in the art in x-rayfluorescence analysis for determining the chemical composition of thespecimens being analyzed. In the invention, the absorption is utilizedtowards determining the filler content of the paper's different layersand thus towards determining the filler distribution. In order that thedetermination of the distribution could be made sufficiently free oferror from the viewpoint of the practical applications, the totalcontents of the different filler components in the paper must be known.This is determined in the invention, by absorption measurements.

If, in the absorption measurements, the attenuation or extinction causedby paper containing filler is measured with two radiation energies whichare as close as possible to the absorption limit of a given component inthe manner that one energy is above and the other below the limit, thedifference in the attenuation or extinction caused by the paper willfurnish information about the content of such filler component. If thepaper contains kaolin, talc, calcium carbonate and titanium oxide asfillers, the difference in the attenuation or extinction of the 5.9 keVline emitted by the ⁵⁵ Fe radiation source and of the K line of titanium(4.51 keV) will furnish information primarily about the titanium dioxidecontent (FIG. 2), the difference in the attenuaion extinction of thedifference of 4.51 keV (Ti K) and 3.69 keV (Ca K) radiations willfurnish information about the CaCO₃ content, and the absoluteattenuation or extinction of the 3.69 keV radiation, primarily about thecombined content of talc and kaolin, these latter components havingabsorption components which at the last-mentioned point are clearlyhigher than the absorption coefficients of any other components of thepaper, as shown in FIG. 2.

In order to determine the contents of various filler components ofpaper, it is necessary to know the base weight of the whole paper, thatis, its mass per unit area in g/m². This is found by measuring theattenuation extinction in the paper of beta radiation for example froman ⁸⁵ Kr source. This is because the different components of paper causeequal attenaution or extinction of beta radiation that is, of electronsthrown out by nuclei. The use of beta radiation for determining the baseweight of paper is known in the art of paper technology and iscompletely routine in its nature.

The fluorescence measurement used for the actual determination of thefiller distribution is described more specifically with reference toFIG. 3, in connection with which it is assumed that the base weight ofthe paper specimen 10 is 100 g/m² and that it contains, as uniformlydistributed filler 25% calcium carbonate. As shown in FIG. 3, theexciting radiation I_(e), in the case under consideration, 5.9 keVradiation from the ⁵⁵ Fe source 20, impinges on the paper specimen 10 atan angle of incidence α and excites in the specimen 10 thecharacteristic radiation of calcium, of 3.69 keV. The detector 30measuring the radiation I_(f) observes the radiation departing at anangle β from the surface 11 of the specimen 10. Since the excitingradiation I_(e) is attenuated as it proceeds in the paper specimen 10,it excites calcium radiation more efficiently in the adjacent topsurface 11, which is closer to the source 20, than in the lower, orback, surface 12. Since the excited characteristic radiation of calciumis also attenuated in the specimen 10 to a given extent, the radiationexcited adjacent the top surface 11 has easier access to the detector30.

Both of the just mentioned act in the direction that the greater part ofthe radiation detected by the detector 30, in the case of homogeneousfiller distribution, comes from the top layers of the specimen 10, andtherefore the topmost layers of the paper will be emphasized in theinformation thus obtained. The smaller the angles of incidence anddeparture α and β of the radiation, the greater are the differences inpath length between the top surface 11 and the lower surface 12, and thegreater is the stress placed on said top 11 in the information gained bythe detector 30. In this manner, it is possible by varying the angles ofincidence and departure α and β, to change the relative weight factorsof different layers in the information that is measured. This isdemonstrated by FIGS. 4A and 4B and by the following Table 1.

                  TABLE 1                                                         ______________________________________                                        Angle of incidence   80°                                                                          30°                                         of the radiation (α)                                                    Angle of departure   80°                                                                          30°                                         of the radiation (β)                                                     ______________________________________                                                      Depth     Intensity                                                                              Intensity                                    ______________________________________                                        Relative intensity                                                                          0.05      0.93     0.86                                         of information from                                                                         0.5       0.47     0.22                                         different depths in                                                                         0.95      0.23     0.06                                         the paper                                                                     ______________________________________                                    

Table 1 shows the relative intensity of the information received influorescence measurements (CaK line; ⁵⁵ Fe source) at various depths inthe specimen when two different pairs of angles of incidence and ofdeparture α, β of the radiation are used. The base weight of the paperis 100 g/m² and its CaCO₃ content is 25%, assumed in this calculationexample to be uniformly distributed in the vertical direction. On thedepth scale, the surface is denoted by coordinate 0 and the back side ofthe paper is denoted by the value 1, making the coordinate of the center0.5.

The intensity values calculated in Table 1 reveal that the informationis strongly weighted in favor of the top side, in other words,emphasizing the side at which the measurement is performed. This effectis considerably strengthened upon changing the angles of incidence anddeparture of 80° in FIG. 4B to the angles α, β of 30° in FIG. 4A. Thisis seen when, for instance, the values of the intensities obtained fromthe center of the paper (0.5) are mutually compared (0.47 and 0.22).

Another manner of varying the relative weighting of the different layersis to use sources with different energies for excitation radiation. If,in the case just considered, the ⁵⁵ Fe source (5.0 keV) is replaced witha ²³⁸ Pu source, of which the strongest radiation components have anenergy 12-17 keV, the attenuation of this in the paper is so minimalthat excitation will occur more or less uniformly throughout thethickness dimension of the specimen 10. For the excited radiation I_(f),of course, the attenuation extinction effects are the same, independentof what radiation has affected the excitation.

If the distribution of a given filler component in the thicknessdirection of the specimen 10 is not uniform but, for example, as shownin FIG. 1, the intensities of the characteristic radiation of calciummeasured on different sides of the paper are unequal and theirdifference reflects the one-sidedness of the distribution. A paperhaving a distribution substantially as in FIG. 1, with a base weight of160 g/m² and a calcium carbonate content of about 20%, yielded with a ⁵⁵Fe source and with angles α, β of incidence and departure, averaging80°, the ratio 470/410 between different sides (top side/wire or lowerside) was found. When the angles of incidence and departure werereduced, the ratio increased, as could be expected. An effect in thesame direction was achieved using a ³ H/TI source, which emits softer(4.5 keV) radiation than the radiation of 5.9 keV of the ⁵⁵ Fe source.

The determination of the filler distribution on the basis of the resultsof measurement will now be considered.

The basic distribution as in FIG. 1, can be mathematically representedby a polynomial y=ax² +bx+c, where y refers to filler content (ordinate)and x to the coordinate in the vertical direction of the paper(abscissa). The coefficients a, b and c are found by fitting to areference distribution. The intensities of the characteristic radiationof calcium are determined from both sides of a paper with referencedistribution to serve as reference values, as is the x-ray absorption ofthe paper with a suitable source for example a ⁵⁵ Fe 5.9 keV, and thebeta absorption such as for example ⁸⁵ Kr source.

Then when the equivalent quantities are measured from an unknownspecimen belonging to the same paper brand, the differences between themand of the quantities measured from the reference paper will yield thefiller distribution of the paper sample being measured, by mathematicalmethods, utilizing the known absorption coefficients of the differentcomponents. In the vicinity of the reference distribution, a measurementcarried out with merely one pair of angles α, β, or with one sourceprovides a rather reliable estimate of the distribution. The reliabilityand accuracy can be increased by varying the angles of incidence and ofdeparture α, β, by using several sources 20 emitting energy I_(e) withdifferent energies. This naturally causes the mathematical processing tobe more complicated.

In a case which was studied, the reference polynomial representing thefiller distribution was found to be y=-42x² +52.1x+6.7, the unit of yand the coefficients a, b and c being the CaCO₃ content in %. It followsthat the CaCO₃ according to the reference distribution is 6.7% on thewire surface 12 (x=0) of the specimen 10, and 16.8% on the top surface11 (x=1).

After the measurement results for the intensity I of the characteristicradiation of calcium where I₁ is the wire side 12 and I₂ is the top side11, and the result of the x-ray absorption measurement T for the paperspecimen under examination have been corrected by applying the referencegraph, with the aid of the results of the beta absorption measurementsto correspond to the base weight of the reference paper, the changes Δa,Δb, Δc of the coefficients of the distribution polynomial for the paperunder examination can be calculated from the system of equationscalculated from the reference polynomial.

    I.sub.1 /I.sub.1 =0.6113·Δa+1,127·Δb+3,344·Δc

    I.sub.2 /I.sub.2 =1.0403·Δa+1,832·Δb+2,781·Δc

    T/T=1/3·Δa+1/2·Δb+1-Δc

In the system of equations, ΔI, IaΔ and ΔT correspond to the values ofthe paper specimen 10 under examination and to those measured from thereference paper.

In tests that have been carried out, the new coefficients obtained fromthe system of equations were found to yield distributions in agreementwith the distributions determined from the same paper specimens byactivation analysis close to the reference distribution. It is obviousthat a more accurate approximation is attained by a greater number ofmeasurements, but the accuracy afforded by the procedure described inthe foregoing is adequate in certain supervision applications.

If in the example under consideration, kaolin, for example, is added tothe filler of the paper in addition to calcium carbonate, as isfrequently done intentionally or inadvertently in reused paper, etc.,the situation is significantly altered in the sense of measuringtechnology. This is because kaolin attenuates, in fluorescencemeasurements, both the exciting I_(e) and the excited I_(f) radiation,especially the I_(f) radiation, and as a result the variations of kaolincontent affect to a certain degree the calcium carbonate measurements,even if the content and distribution of the calcium carbonate should beconstant in the specimen 10. The influence of kaolin on the results ishowever calculable and can be eliminated with the aid of the knownabsorption coefficients, provided that the kaolin content in thespecimen 10 is known. This leads to the requirement of measuringtechnology that, in connection with the measurements the contents ofkaolin and other potential filler components have to be determined. Thisis possible by using suitably selected radiation energies in theabsorption measurements, as hereinbefore described. It may be observed,in this connection, that of the commonly used fillers, talc and kaolinare materials of which the contents must be determined by the absorptiontechnique. Fluorescence measurements do not succeed in normal conditionsbecause in these substances the characteristic x-ray radiation, even ofthe heaviest element, silicon (Si), is so weak that it is excessivelyattenuated in the specimen 10, in the air space and in the windows ofstandard detectors 30. The same methods applied for CaCO₃, may beapplied for TiO₂, which is occasionally used with the difference, ofcourse, that the K line (4.51 keV) of titanium is excited and measured.

It is thus understood that in complicated cases the determination of thethickness-direction distribution of filler requires several x-rayfluorescence measurements on both sides of the specimen 10 and severalabsorption measurements. The intensity of the exciting radiation I_(e)scattered back from the specimen 10, which correlates with severalcharacteristics of the specimen paper may be used as a kind of controlquantity in the measurements. In practice, when one is moving quiteclose to a given reference distribution, adequate accuracy is oftenachieved with rather few measurements.

The printing characteristics of paper can be improved by coating thepaper with the same substance that are used as fillers. In this case,the contents of mineral components in the surface layers of the paperincrease greatly, as seen in FIGS. 5A and 5B. Since the method of theinvention provides information about the distribution of the mineralcomponents in the paper and, in particular, about the content in thesurface layers of the paper, it is also possible to determine the amountof coating in the coating layers and the difference in coating betweenthe different sides of the paper by the method of the invention withoutdestroying the specimen. If the paper is already coated, the fillerdistribution of the uncoated bottom paper naturally cannot be elicitedany longer.

The part isolated by interrupted lines in FIGS. 6A, 6B, 6C and 6D is themeasuring head 100. The measuring head 100 comprises radiation sources20 and their transporting mechanisms 22 known in themselves in the art,radiation transformation plates and their transfer mechanisms 22 (notpresented in detail), a radiation detector 30 and a pre-amplifier 31. Inlaboratory apparatus, the measuring head 100 is, for example, anenclosed apparatus on the table, into which the paper specimen 10 to beexamined is conveyed by a suitable mechanism, which transports the paperduring one measuring cycle into one or several measuring positions.

In an in-line apparatus performing the measurements directly on thepaper machine, the paper web 200 of FIG. 7 passes through the measuringhead 100 mounted on a measuring beam 300. The measuring head 100 may beso constructed that it may traverse the paper web, as shown in FIG. 7.

The detector 30 consists of a proportional counter. In certaininstances, in particular, in laboratory measurements, a semiconductorcounter may also be used with a view to increasing the accuracy.

The measuring head 100 is connected to measuring apparatus 40 comprisinga voltage source 41, an amplifier and a counter, processor and displayunit 42. A control unit 43 connected to the processor governs theperforming of the measuring cycle and the processing of results.

In the laboratory version of the means of the invention, the processorfunctions may be replaced by manual operations, and the results may, ofcourse, be processed manually or by an external computer 50. In fact,the measuring equipment external to the measuring head 100 is standardmeasuring equipment, and the inventive idea proper is associated withthe measuring head 100.

The extent of the equipment external to the measuring head 100 and ofthe computer 50 software and programs is greatly dependent upon thedegree of automation and the standard of accuracy desired, and, theextent of the measuring range, that is on the number of different paperbrands and the variation limits, within each brand, of the quantitieswhich are measured.

FIG. 6A illustrates the exciting of the characteristic fluorescenceradiation of a filler component CaCO₃ or TiO₂ and its measuring at theother side of the paper specimen 10. The radiation emitted by theradiation source 20 excites in the specimen 10 the characteristic x-rayradiation of a given element (Ca or Ti) of a filler, part of which isdirected to the detector 30 and counted. The detector 30 differentiatesbetween the different types of radiation by their energy with suchaccuracy that the contribution of each radiation component can bedetermined by mathematical means from the measured pulse heightdistribution. If it is desired to make the measurement at excitingradiations I_(e) having different energies, the source of radiation 20may be exchanged with the aid of a suitable mechanism. If, again, it isdesired to utilize different angles of incidence and departure, α,β ofthe radiation with reference to the surface of the paper specimen 10, itis possible to move the source 20 laterally and to use appropriatecollimators or radiation beam detectors, known in the art.

FIG. 7 is a schematic diagram of a measuring head 100 mounted on atransverse measuring beam 300 in a paper machine to perform on-linemeasurement traversing reciprocatingly the width of the travelling paperweb 200.

Since for determining the filler distribution, a fluorescencemeasurement has to be made at both sides of the specimen, in thelaboratory version, the paper specimen 10 must be turned over, or twomeasuring heads 100 carrying out measurement at different sides of thespecimen 10 have to be used. When measurements are carried out directlyin the paper machine, the only possible alternative is the two heads.

FIG. 6B presents an arrangement by which absorption measurements arecarried out with x-ray radiation of different energies. The radiationfrom the radio isotope source 30 is either scattered by an exchangeabletarget plate 21 or excites therein radiation appropriate for absorptionmeasurements, such radiation passing partly through the paper specimen10 into the detector 30. The 5.9 keV (⁵⁵ Fe), 4.51 keV (Ti K) and 3.69keV (Ca K) radiation components which were hereinbefore required in theabsorption measurement example are obtained with the aid of a ⁵⁵ Fesource by means of plastic or scattering, titanium or marble targetplates 21. In specific instances, the absorption measurements, too, maybe performed with the aid of the source used in the fluorescencemeasurements, as shown in FIG. 6C.

As shown in FIG. 6C, the radiation that has passed from the source 20through the specimen 10 is scattered from the backing plate 21 orexcites therein radiation appropriate for absorption measurements, whichradiation partly passes through said specimen into the detector 30. Inthis instance, the signal of the radiation excited in the paper specimen10 by the source 20, which signal reduces the accuracy of measurement incertain cases is admixed with the signal being measured.

FIG. 6D presents the absorption measurement, apparatus of the invention,used in routine in the papermaking industry for base weight measurementsand carried out with the aid of a beta radiation source 23. Thismeasurement provides the auxiliary quantity which is indispensable inthe processing of the results of measurement in distributionmeasurements according to the invention.

Detailed reference distributions which are indispensable fordemonstrating and proving the practical applicability of the method ofthe invention may be determined by neutron activation analysis ofmicrotome sections made of paper. The technique is described in anarticle by Kuusi, J. and Lehtinen, A. J. entitled "Neutron ActivationAnalysis of Microtome Cuts in Examination of Paper for Its FillerDistribution", Pulp and Paper Magazine of Canada, 71, No. 3 (1970).

The method and apparatus hereinbefore described are suitable for useeither in laboratory measurements or on-line measurements in a papermachine. In the on-line measurement use, the results obtained by themeasuring apparatus may be used as feedback signals for guiding and/orcontrolling the papermaking process towards implementing a desiredfiller distribution. A possible application of the invention is the useof the method or apparatus in the measurement, and possibly even in thecontrol, of the coating agent content and.or coating distribution eitherof paper or cardboard to be coated in an on-line process, or of papertreated in separate coating means, in particular of its one-sidedness.Further applications of the invention may be the quality control ofpaper fed into a printing press, and even the guiding control of theoperation of a printing press for optimizing the printing press.

The invention is by no means restricted to the aforementioned detailswhich are described only as examples; they may vary within the frameworkof the invention, as defined in the following claims.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above constructions withoutdeparting from the spirit and scope of the invention, it is intendedthat all matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

I claim:
 1. A method of measuring, without destroying the specimen, thedistribution in the thickness direction of the filler and/or coatingmaterials of paper or cardboard, and the content of said materials,wherein radiation emitted by a radio-isotope source is used to excite inthe material component to be examined, of the object of measurement, itscharacteristic X-ray fluorescent radiation, the intensity of saidcharacteristic radiation being observed, measurements are made on bothsides of the paper or cardboard under examination, the contents offiller components are determined by X-ray absorption measurements foreliminating the effects of these components disturbing the distributionmeasurement, and measurement is made of the base weight of the paper orcardboard under examination, in g/m², by radiation absorptionmeasurement, said method comprising the steps ofmaking a number of X-rayabsorption measurements, the number of measurements being at least equalto the number of different filler components which are to be consideredseparate from the point of view of X-ray absorption measurements, fordetermining the contents of the different filler components and thecoating materials by X-ray radiation; making a number of measurements ofthe characteristic X-ray fluorescent radiation of the materialcomponents excited by a radioisotope source and determining thedistribution of fillers and coating materials by calculative jointprocessing of the results from said measurements.
 2. A method as claimedin claim 1, wherein radiation is obtained directly from said source. 3.A method as claimed in claim 1, wherein radiation is provided by use ofa transformation target.
 4. A method as claimed in claim 1, furthercomprising the eliminating the effects exerted on the fluorescentmeasurements by variations of the different filler components' contentsrelative to each other by calculation with the aid of the total contentsof the different filler components determined by X-ray absorptionmeasurements in a manner whereby the thickness direction distributionsof the different filler components in the paper or cardboard isdeterminable from the fluorescence measurements.
 5. A method as claimedin claim 1, wherein the distributions of different material componentsare measured by the use of radiation sources with different energylevels (E), each energy level (E) being selected so that it is slightlyhigher than the Kα absorption limit of the material component to beexamined.
 6. A method as claimed in claim 1, further comprising the stepof determining the intensity of the radiation from said radio-isotopesource scattered back from the paper or cardboard, which correlates withthe base weight of the same, so as to provide an auxiliary quantity inthe processing of results, in addition to X-ray fluorescencemeasurements.
 7. A method as claimed in claim 1, wherein the contents ofvarious filler components are measured by X-ray absorption measurementsutilizing the primary radiation emitted by said radiation source andradiation with certain adsorption properties derived from said source.8. A method as claimed in claim 1, wherein the contents of variousfiller components are measured by X-ray absorption measurementsutilizing the primary radiation emitted by said radiation source andradiation from a source placed on the other side of said paper orcardboard via a transformation target.
 9. A method as claimed in claim1, wherein the filler material of said paper or cardboard examination isprincipally kaolin, talc, calcium carbonate and/or titanium oxide, saidmethod utilizing 5.9 keV radiation emitted by a ⁵⁵ Fe radiation sourceas the primary radiation source and that of the characteristic 4.51 keVK line excited in titanium primarily in determining the titanium dioxidecontent, utilizing the absorption difference observed between said Kline of said titanium and the 3.69 keV K line of calcium primarily indetermining the CaCO₃ content, and using the information provided by theattenuation of said calcium K line primarily for determining thecombined content of talc and kaolin.
 10. A method as claimed in claim 1,further comprising the step of measuring the attenuation in the objectunder measurement by beta radiation emitted by an ⁸⁵ Kr source todetermine the base weight in g/m² of said paper.
 11. A method as claimedin claim 1, wherein said measurements are carried out at at least twodifferent angles of incidence (α) of said radiation.
 12. A method asclaimed in claim 11, wherein said measurements are carried out at atleast two different angles of departure (β) of the characteristic X-rayradiation excited in the specimen by said radiation.
 13. A method asclaimed in claim 12, wherein the angle of incidence (α) of saidradiation is equal in magnitude to the angle of departure (β) of theexcited radiation relative to the plane of said paper or cardboard onthe same side of the same.
 14. Apparatus for measuring the distributionin the thickness direction of filler and/or coating materials of paperor cardboard, and the content of said materials, without destroying thespecimen, said apparatus having a radio-isotope source providingradiation used to excite the characteristic X-ray radiation of thematerial component under examination, of the object of measurement,means for observing the intensity of said characteristic X-rayradiation, means for performing measurements on both sides of the paperor cardboard and for determining the contents of filler components byX-ray absorption measurements for eliminating the effects of thesecomponents disturbing the distribution measurement, and means formeasuring the base weight, of the paper or cardboard under examinationby radiation absorption measurement, said apparatus comprisingameasuring unit having a power source, an amplifier, a counter, aprocessor and a display unit; and a measuring head connected to saidmeasuring unit, said measuring head having radiation sources, transfermeans for said radiation sources, a radiation transforming plate,transfer means for said plate, a radiation detector and a preamplifierconnected to each other in a manner such as to perform absorptionmeasurements for the determination of the contents of different fillercomponents by utilizing radiation excited from different radiationsources.
 15. Apparatus as claimed in claim 14, wherein said measuringunit further comprises a control unit which controls the measuring cycleand the processing of measurement results.
 16. Apparatus as claimed inclaim 14, wherein said radiation detector in said measuring headcomprises a proportional counter.
 17. Apparatus as claimed in claim 14,wherein said radiation detector in said measuring head comprises asemiconductor counter.
 18. Apparatus as claimed in claim 14, furthercomprising a computer connected to said measuring unit, said computerbeing programmed with a measurement result-processing and outputtingprogram.
 19. Apparatus as claimed in claim 14 wherein said measuringhead is disposed to traverse reciprocatingly over the entire width ofthe specimen or part thereof.